In the distribution of electrical energy, electric utility companies have typically found it desirable to not only measure real load energy as watthours delivered to a user, but also, reactive load quadergy as varhours (or reactive volt-amperehours), apparent power and power factor (cos .theta.). By measuring both watthours and varhours, electric utilities can more accurately apportion the costs of supplying energy to those customers who demand the most from the power delivery network.
In the past, the measurement of varhours was either accomplished by using a second meter in conjunction with a conventional watthour meter or, more recently, a meter with the built-in capability of measuring both watthours and varhours, such as a General Electric.RTM. Phase3.TM. Electronic Polyphase Meter. In either case, however, the technique for measuring varhours has typically required the phase shifting of the measured line voltage by 90.degree. (1/4 cycle) or cross-phasing of line voltages and currents. Voltage phase shifts of 60.degree. result in a different measurement generally called Qhours. Qhours are a reactive measurement that evolved from the fact that a 60.degree. phase shift could be readily accomplished by cross-phasing the meter voltage connections to a polyphase circuit at the meter, thus eliminating the need for phase shifting transformers as is required for the measurement of varhours.
However, providing a 90.degree. phase shift, which typically involves delaying the line voltage by a fixed time interval (5 ms for a 50 Hz rated meter), will cause errors in the measured varhours if the line frequency deviates from the rated frequency of the meter. These errors are unlikely to be appreciable in the United States and Western Europe, but in many other countries, electric utility companies have greater difficulty guaranteeing that the line frequency will be maintained within a predetermined range about the rated line frequency.
Moreover, since many of these countries also have strict regulations limiting the variations in acceptable error for meters of various classes, it is necessary that meters be capable of detecting line frequency variations and correcting measured varhours, to more accurately reflect the actual quadergy. For example, the international standard set by the International Electrotechnical Commission (IEC), section 1030, applicable to meter accuracy classes 2 and 3, requires that for a reactive power factor (sin ) of either 0.5 or 1.0, the variation in error must be .ltoreq.2.5%, given a frequency variation of .+-.5%.
Accordingly, there is a substantial international need for meters capable of accurately measuring varhours even when line frequency variations are appreciable.